Controlling the Amount of Copper Formate Shells Surrounding Cu Flakes via Wet Method and Thermo-Compression Sinter Bonding between Cu Finishes in Air Using Flakes

To bond a semiconductor power chip on a substrate using the formation of bondlines exhibiting long-term mechanical durability at high temperatures such as 300 °C, compression-assisted sinter bonding between Cu finishes was carried out at 300–350 °C in air. Cu flakes of approximately 7 µm were used as the main filler material; their surfaces were modified by pretreatment using a formic acid solution, and the existing oxide layers were transformed into copper formate shells. To increase the amount of the copper formate shells, the shell transformation reaction was controlled by the deliberate addition of 30 nm Cu2O particles and sustained for 50 min. The formate shells formed decomposed at a peak temperature of 250 °C, forming pure Cu, which rapidly induced sintering between the flakes and at the Cu-finish–flake interfaces. Therefore, the paste containing Cu flakes showed a sufficient shear strength of 26.3 MPa even in air after sinter bonding for only 3 min under 5 MPa at 350 °C. Although the bimodal-type paste presented near-full-density bondline structures within 3 min of adding 350 nm pure Cu particles, the strengths with respect to bonding time showed trends of values less than 26.3 MPa, owing to the excessive oxidation of the 350 nm Cu.